This invention relates generally to active filters and more particularly to low-pass and high-pass active filters which are electronically controllable over a large frequency range.
Networks using active elements, such as operational amplifiers, and only resistors and capacitors, permit the realization of circuits with responses characterized by having complex, left-hand, s-plane poles. The location of these poles in the complex s-plane and hence the frequency response of these networks is determined by the values of the R and C components used. This makes such circuits useful as active filters, while obviating the need for inductances which are bulky and sometimes difficult to realize practically. This is a significant advantage. For example, active filters operating at low frequencies can be physically quite small and lightweight in comparison to conventional tuned filters employing LC resonant circuits. Furthermore, since the response of such filters can be varied by changing the values of the RC components, they can be tuned relatively easily. For example, U.S. Pat. No. 3,607,567 to Webb, discloses an active filter employing a voltage amplifier in which a feedback network interconnects the amplifier output and a passive RC network. By regulating the values of the load resistor and capacitor, the location of complex poles relative to zeros produced by the active filter, can be adjusted separately and independently, thereby providing different frequency-selectivity characteristics. Different frequency characteristics may also be realized by adjustment of the amplifier gain.
The ability to adjust the frequency response of an active filter by selecting the values of the R and C frequency determining elements makes active filters particularly adpatable to tuning. For example, tuning may be accomplished by mechanically varying the values of the frequency determining elements, such as by using variable capacitors and potentiometers. This makes active filters particularly useful in signal processing applications, where it is desirable to be able to tune a filter to accommodate different input signals. Active filters are also adaptable to electronic tuning, and are useful where the signal environment is dynamic and filters must be quickly and easily tuned to optimize processing.
There are various ways of electronically tuning active filters. One such method is disclosed in U.S. Pat. No. 3,528,040 to Galvin, in which photo-sensitive resistors are used in the frequency determining network. The resistance of these photosensitive resistors is a function of the intensity of the light which is being used to illuminate them. Therefore, by controlling the light's intensity, their resistance value can be varied and hence the filter can be tuned.
Another method of electronically tuning an active filter is illustranced in Graeme, J. G., Designing With Operational Amplifiers, Burr-Brown Electronic Series, McGraw-Hill, New York, 1977, pp 118-121. A multiplying D/A converter is employed in a feedback loop around the active filter's operational amplifier, to multiply the feedback voltage by a factor which is proportional to the value of an input digital word. This causes the voltage impressed on a frequency determining element to be increased by the same factor, thereby increasing the current through that component. The component's value is thus effectively divided by the multiplication factor. This results in changes in the circuit's time constants and hence controls the filter. Since the multiplying D/A converter is included in a feedback loop, control is achieved only if the gain constant of the multiplying D/A converter is positive. Otherwise, the polarity of the feedback voltage would be incorrect and the circuit would become unstable. In place of a multiplying D/A converter, a 2-quadrant or 4-quadrant multiplier could also be used to accomplish the same multiplying effect.
The control of many active filters is complicated by the fact that in some cases there is an interaction between the frequency-determining networks. This requires that they be isolated with buffer amplifiers. Furthermore, the filter's damping factor determines the filter's characteristic. Since the damping factor is a function of the ratio of the filter-component values, where the ratio of these components is changed, as when the filter is tuned, the shape of the filter can also change. This makes it difficult to maintain constant a desirable filter characteristic and to exercise precise control over the filter characteristics as it is tuned over a large range.
There is another disadvantage that is encountered in using active filters, particularly low-pass filters which are DC coupled. Active filters typically use operational amplifiers as their active elements. Since these amplifiers have bias and/or bias-leakage currents, they are subject to DC voltage offsets due to the resultant voltage produced by these currents flowing through circuit resistances. In addition, these amplifiers typically have a differential offset voltage through them. These DC offset voltages can change as the filter is tuned. Furthermore, these DC offset voltages can limit the dynamic range of the filter since they can become significantly large compared to the signal of interest.
It is desireable therefore to provide a new and improved, electronically controllable active filter which overcomes the aforesaid disadvantages, and it is to this end that the present invention is directed.
Accordingly, it is an object of the invention to provide an electronically controllable filter which is tunable over a large frequency range, while maintaining a fixed-filter characteristic and a constant, near-zero DC offset voltage.
It is a further object of the invention to provide a filter which may be cascaded with other filter sections to provide any desired number of poles or zeros.
It is also an object of the invention to provide a filter which may have any desired filter-response characteristic, i.e., Butterworth, Bessel, Chebyshev, etc.
It is additionally an object of the invention to provide a filter which may be tuned without changing the component values of the frequency-determining elements and which is tunable over a 50:1 or greater frequency range.
These and other advantages may be obtained in an active filter which has at least one passive element-multiplier frequency-determining network connected to the input of a first amplifier, which amplifier has a feedback path connected to its output and the passive element-multiplier network, for feeding back a portion of the output of the first amplifier to the passive element-multiplier network.